The present invention relates to a method of treating water in a water cooling system, pulp manufacturing process, and so on, in which microbial fouling on a metal pipe and the like contacting water is monitored and an appropriate water treatment is carried out based on the result of monitoring. More particularly, the present invention relates to a water treatment system in which microbial fouling on a metal pipe and the like is predicted early and precisely by monitoring change of corrosion potential of a sensor having a sensitized metal and contacting the water, and thus an appropriate water treatment is carried out based on the prediction.
Some problems of a water system are associated with slime. Slime which is formed by microbes in water causes decreasing a heat transfer efficiency of a heat exchanger, clogging of pipes, and corrosion of pipes and the like. To prevent such problems, various slime control treatments are done; for example, a microbicide is added into the water system. Moreover, according to an amount of slime adhered on the pipes, a slime removing agent is added into the water to eliminate slime adhered on pipes and the like (xe2x80x9cManual for Protecting Pipes from Corrosionxe2x80x9d, edited by Kajima Construction Co., Ltd. and Kurita Water Industries Ltd., and published by Nippon Kogyo Publishing Company in 1987).
According to a conventional method of monitoring microbial fouling in a water system, a rubber plate is immersed in water of a water system, and the rubber plate is measured periodically on an amount of a fouling component adhered to the rubber plate. Another method is disclosed in NACE Standard RPO189-89, xe2x80x9cStandard Recommended Practice On-line Monitoring of Cooling watersxe2x80x9d, NACE International, Houston, 1996, in which microbial fouling is detected by a change of pressure difference in a tube.
In natural seawater, stainless steel becomes to have an extremely high corrosion potential when microbes adhere thereto (R. Johnsen, Corrosion, 41:296, 1985). The same phenomenon occurs in a cooling water system (Hirano et. al: 38th corrosion-corrosion prevention debate, 1991).
Japanese patent publication H6-201637A and Japanese Patent 2794772 disclose methods of monitoring microbial fouling by measuring a natural electrode potential. Japanese patent publication H10-142219A and Japanese patent publication 2000-9674A disclose a method of controlling addition of an agent based on measured results of a corrosion potential.
In a conventional slime control treatment, an appropriate slime control agent is selected for a water system and an amount of the agent to be added is predetermined or controlled so that the concentration of the agent in the water system would be in a predetermined range. Lately, a slime control treatment has been carried out with using a controlling apparatus to meet the requirements of ecology, safety handling, better workability, and so on. However, because of various external factors such as the deterioration of water quality, a sufficient treatment may not be accomplished by adding usual amounts of agents or by controlling concentrations of agents in the water system. Then, microbial fouling adhere to the system and cause various problems.
In order to prevent such problems, more amount of a slime controlling agent may be added according to a fouling level of a water system which is measured by monitoring adhesion of contaminants. It is desirable to monitor the adhesion of fouling all the time and to add more amount of slime controlling agent soon after the adhesion of fouling is detected. However, at the present time, the slime controlling agent is increased in its additive amount manually based on the fouling level monitored from time to time.
It is desirable to control the addition of agents automatically based on the result of on-line monitoring. However, the aforementioned method using a rubber plate can not be an on-line monitoring method.
The other aforementioned method measuring differential pressure of a tube can be an on-line monitoring method. However, a fouling level measured as a change of the differential pressure is considerably affected by the way of adhesion of fouling elements to a tube, that is, whether the fouling elements adhere to the tube evenly or unevenly. Furthermore, there is an disadvantage to install a monitor of this type in each cooling water system. In addition, expensive parts such as a differential pressure gauge must be equipped and a monitoring apparatus needs a large space to be installed because the tube needs enough length for measuring differential pressure. Since a differential pressure should be measured under a condition of a constant flow velocity of cooling water in a tube, the water flow in the tube is adjusted by a constant flow valve or the like. An obtained value of differential pressure is often affected by a small change in the flow velocity caused by fouling in the valve.
The surface of a plate sensor used in a conventional method of monitoring the corrosion potential is so smooth that its sensitivity is not good enough.
As mentioned above, there are a lot of problems in each conventional method in which the addition of agents or the operation of an apparatus is controlled based on the monitoring result of microbial fouling.
When a slime control treatment is not accomplished sufficiently and a large quantity of slime adheres to a water system, the slime should be removed off. However, when the water system is not monitored all the time, the fouling level in the system is unclear and the slime is retained in the water system for a long time.
When slime is removed from the water system by adding a removing agent automatically at regular intervals controlled by a timer, the water system is saved from fouling. However, in this case, the removing agent may be added even if there is no fouling. That is a waste of the agent. A necessary treatment should be done at a necessary time to reduce treatment cost and to meet the ecological requirements.
It is an object of the present invention to provide a method of water treatment in which microbial fouling onto metallic pipes and the like is detected early and precisely and an appropriate water treatment is carried out according to the detection.
A method of water treatment in a water system of the present invention employs at least one sensor having a sensitized metal piece for monitoring microbial fouling, and the water is treated according to a change of electric potential of the metallic sensor.